if you're wanting the sharpest crispest results, a 17mm prime will be sharper than both of these zoom lenses.
If you desparately need that wider 12mm end than the 12-24 option seems the logical choice. Which other lenses do you have for the 5D? If you have the 24-105 kit lens, than this choice would seem even more logical.
Otherwise (if you don't have the 24-105), I'd personally go for the 15-30 if you had to choose between just the two lenses.
If you're really unsure, why not take your camera and memory card into a shop and try both?....Or, go to hire from somewhere like Calumet..you might not be able to hire those specific lenses, but you could hire one that covers those wide angles...which would help you decide whether you really need or want to go as wide as 12mm wide.
P>S 12mm full frame is massively wider than 10mm on APS-C.
hope this helps?
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